This application relates in general to the control of power MOSFETs (metal-oxide-semiconductor field-effect transistors) switching modules and, in particular, to a method and a device for controlling a commutation process of a load current from a first power MOSFET switching module to a second power MOSFET switching module.
Power semiconductor switches are used in various technical fields, e.g. in converters for converting electrical energy and for controlling the energy flow, such as for electrical machines, motors, generators, in power converters for power transmission, conversion and other applications. They can be used for switching high voltages, up to a few kV, at high switching frequencies, up to the high kHz range. While insulated-gate bipolar transistors (IGBTs) have become established in particular for regulated drives, power supply and transmission applications, metal-oxide-semiconductor field-effect transistors (MOSFETs) are at high switching frequencies and lower voltages. In view of rising energy costs, increasing environmental awareness and intense competitive pressure, there is a continuing desire for new power semiconductor switches and power modules formed from them that are more compact in terms of their design, have lower inductance, are less expensive and more efficient to produce, and that enable improved performance, power density, and efficiency.
For conventional switching applications, the power semiconductor switches must be able to accept a reverse voltage. Due to their structure, standard IGBTs are only able to supply voltage and carry current in one direction. Therefore, a separate free-wheeling diode is usually connected in antiparallel to the IGBT device that can accommodate the corresponding blocking voltage and permits current to flow in the reverse direction. This usually increases the construction volume, construction cost, connection technology and chip area and thus the cost of the switching module.
IGBTs are also known whereby a free-wheeling diode is monolithically integrated and these are referred to as reverse-conducting IGBTs (or RC-IGBTs for short). While the conduction behavior of a separate diode connected in antiparallel to the IGBT is independent of the control status of the IGBT, the integrated diode of an RC-IGBT shows a dependence on the control status of the IGBT. If the RC-IGBT is switched on in reverse operation, when the intrinsic-body diode carries current, an additional current path parallel to the intrinsic-body diode results, such that not all electrons can contribute to the flooding of the intrinsic-body PIN diode structure. This can lead to an undesired, increased voltage drop across the diode or the IGBT in reverse operation that results in increased semiconductor losses.
To prevent this, DE 10 2009 001 029 A1 proposes a device and a method for controlling an RC-IGBT that generate control signals to control the gate electrode to switch the RC-IGBT on and off according to a control signal, whereby switching on the RC-IGBT is blocked when the IGBT is conducting in the reverse direction, thus requiring current direction detection for the control and operation of the device.
Due to their bipolar nature, IGBTs have a tail current and show slower switching performance compared to unipolar devices such as MOSFETs. The blocking and reverse recovery time of a bipolar assembly and the comparatively higher switching energies caused by majority and minority charge carrier ratios thermally limit the maximum switching frequency. As a result, for applications with high switching frequency requirements, MOSFETs are increasingly used at higher voltages above 1,000 V. In addition, a threshold voltage does not occur in a MOSFET. The throughput behavior and conduction losses are determined in the MOSFET by an ohmic behavior that is characterized as resistance RDS(on). RDS(on) consists of different resistance components consisting of the channel, drift zone, enhancement layer, substrate, and other smaller quantities. In particular, more recently developed silicon carbide (SiC)-based MOSFETs offer significant switching speed advantages over conventional silicon-based IGBTs and the maximum depletion layer temperature. SiC MOSFETs benefit from a wider band gap, permitting higher critical field strength for comparatively thinner devices that reduce material costs and throughput losses. In addition, the larger band gap permits higher operating temperatures that in turn permit improved utilization of the devices, increase power density, and reduce the cost of the semiconductor cooling system. The development of SiC MOSFETs is not yet technologically complete, meaning that appropriate control strategies can offer additional advantages.
As far as MOSFET devices are concerned, if reverse conductivity is required, their intrinsic-body diode can be used. As a double-diffused structure (DMOS), the MOSFET forms an intrinsic-body diode, also referred to as an inverse diode. In this structure, it typically forms a PIN diode that permits current to flow in the reverse direction from the source-connected substrate, across the midregion to the drain contact. Compared to an optimized, discrete free-wheeling diode that is usually adapted to the IGBT or selected accordingly, the intrinsic-body inverse diode of the MOSFET is inferior in terms of its switching behavior and usually results in increased switching losses due to higher storage charging and possibly larger reverse current spikes. Intrinsic-body diodes are usually not or only partially optimized, whereby the distribution of the charge carriers in the device and their dynamics during switch-off can result in high-frequency oscillations and snappy switching behavior, which must be compensated or optimized by appropriate wiring, adjustment of the control or by an appropriate design of the semiconductor structure. However, the use of the inverse diode of a MOSFET permits a reduction in the required space, the chip area used and the structure and connection technology of the switching module. In conjunction with the unipolar structure of a MOSFET, it also permits a possible reduction of the reverse recovery time and consequently higher switching frequencies. For applications that are primarily intended for power converter operation, the saving of a discrete, additional diode represents a significant cost advantage that can be considered for the particular application and its requirement. For inverter mode applications, diodes are only little used compared to switching elements, which permits the saving of discrete diodes when using MOSFETs.
One disadvantage of MOSFETs, including SiC MOSFETs, is their strong dependence on the switching behavior of the parasitic circuit parameters. In particular, a MOSFET switching module during switch-off, when commutating the current flow from an inverse diode, tends to cause strong oscillations due to the leakage inductances and parasitic capacitances of the switching module in conjunction with the very short rise times and possible snappy switch-off behavior due to an abrupt reverse current break of the inverse diode. The oscillations during and after the switching transients result in increased switching losses, high voltage peaks and stronger electromagnetic radiated interference. The latter require expensive measures to improve the electromagnetic compatibility (EMC) in terms of robustness and emitted interference, with correspondingly increased manufacturing and component costs.
The current-carrying capacity of SiC MOSFETs is relatively limited. In applications with higher currents, therefore, several MOSFET switching modules must be connected in parallel to achieve the required current-carrying capacity. The oscillations of the commutating intrinsic-body diodes have a particularly disadvantageous effect here, because these oscillations of the parallel-connected inverse diodes can mutually influence and amplify each other. A reduction in these oscillations would be extremely important for a parallelization of the switching modules.
A common method of reducing such oscillations of inverse diodes of MOSFET switching modules to switching transients consists of increasing the gate resistances in the switching process. For example, the dissertation by Li, Helong “Parallel Connection of Silicon Carbide MOSFETs for Multichip Power Modules”, Department of Energy Technology, Aalborg University, Denmark, 2015, describes the impact of increased switch-on and switch-off gate resistances on the switching behavior of SiC MOSFET switching modules. By increasing the gate resistance, the oscillations can be reduced, whereby the switching losses however increase. In general, however, there is a desire to minimize both the throughput and the switching losses.
In the publication by Zhenxue Xu et al. “An Analysis and Experimental Approach to MOS Controlled Diodes Behavior”, IEEE TRANSACTIONS ON POWER ELECTRONICS, Vol. 15, No. 5, September 2000, different inverse diodes of power MOSFETs are studied, and it is described in terms of their behavior, how the storage charging and the reverse current peak of the inverse diodes for the commutation can be reduced by appropriate control of the MOSFET. In this case, the component is generally used as a diode or in the diode current direction in a step-down converter circuit. For commutation, the entire current is conducted across the unipolar channel of the MOSFET opened by UGS, while the channel for conduction mode is closed and the bipolar structure of the PIN diode alone is effective. For open-channel commutation, the component's throughput voltage is lower than the intrinsic-body diode threshold voltage and the unipolar channel of the MOSFET is used for commutation, thereby reducing the storage charging and the return current spike.
DE 103 23 445 B4 describes a clocked DC control circuit for commutating a current flow between two controlled power components, in particular MOSFETs, that are connected and operated in the manner of a step-down converter circuit. In particular, one of the two power components is connected as a switching element, while the other power component is connected as a free-wheeling element. After initiating the switch-off operation of the switching element, the switch-on of the free-wheeling element is initiated, whereby the switch-off of the switching element is delayed and then the switching element is completely switched off and whereby the switch-on of the free-wheeling element is delayed during a predetermined time period during the complete switch-off of the switching element and is then switched on completely. By using the controlled free-wheeling element, it is achieved that it is not the relatively slow and interference-critical intrinsic free-wheeling diode of the switching element that determines the commutation, but rather the other power component that is arbitrarily rapidly switchable and serves as a free-wheeling element. Due to the active control of the free-wheeling element separate from the switched switching element, losses can be minimized without generating additional emitted interference.